AC Motors, VFDs, and Energy Savings at Toronto General Hospital

I’d like to share two recent energy projects at Toronto General Hospital, which used VFDs to achieve significant energy savings and improve control. When I say significant I mean that the savings could buy a house, well maybe not a house in Toronto. There is also a fairly technical explanations of motors and variable frequency drives but I’ll leave that to the end for obvious reasons.

VFDs and Harmonic Filters on Secondary Chilled Water Pumps at TGH

Project 1: Reinstate VFD operations on Air Handling Units

At TGH we had 17 old non-functional variable frequency drives (VFDs) that are used to control motor speed (you’ll have to read the technical section if you want more explanation of what a VFD is and how it works). Most of the drives were on large fans, most of them old, some from the early 1990s. They had put in their time but like all electronics had eventually worn out beyond repair. We replace these VFDs and observed the following change in energy use (based on 2 months of data in May/June, projected out to 1 year).

That’s 55% energy savings!

When we started the project the existing fan motors were operating at constant speeds and using original failing inlet vanes from 1980 to control the flow. Inlet vanes are like air brake. A common analogy equates this type of control to driving your car with your foot fully down on the accelerator and then controlling the speed with the brake. It works but it’s not very efficient. The inlet vanes are also pneumatically driven; highly prone to failure and/or falling out of calibration.

The total connected horsepower is big, 940 hp and the fans run 24/7 to serve the hospital. For the project we replaced old VFDs, fixed the wiring (line and load were in the same conduit = bad for harmonics), added harmonic filters, added load filtering to protect the motors, replaced pressure sensors, and decommissioning the old inlet vanes. Load filtering with LC line rectifiers were used because some of the existing motors have Class B insulation. Typically motors with Class F or better insulation are recommended to prevent the high voltage spikes from the VFD from breaking down the motor insulation and arching in the windings (i.e. frying the motor). LC rectifiers work by slowing down the voltage spike with several windings of copper and then cutting off that peak voltage by storing the charge in capacitors.

The project had a payback of 7.5 months and will improve the ability to control ventilation and temperature in the hospital.

Project Cost

$ 300,000

Hydro Incentives

$ 150,000

Annual Energy Savings

$ 245,000

Project Payback

0.61 Years

Project 2: Secondary Chilled Water Pump VFDs

The secondary chilled water system provides chilled water to all 2 million square feet of Toronto General Hospital. This system keeps the air cool and also provides cooling to critical processes and equipment in TGH, like the cyclotron, nutrition fridges, some CTs and MRIs, etc. For reference the chilled water plant’s peak chilled water consumption is ~4100 Tons and the pumps move around 6000 gpm at peak flow. There are four 60HP pumps that provide the chilled water through six different risers (i.e. large pipes providing cooling to each building). The pumps operate at constant speed and were enabled/disabled manually as needed.

There’s a lot of overlap in the number of pumps running to achieve the same amount of cooling. VFDs will help that.

There’s room for improvement here, but this is already a big improvement over the constant speed case above.

We were lucky in that within each building most of the control valves on the cooling coils are two way valves, which cut off the flow when not needed. That’s good for VFD control but unfortunately also means that for constant flow systems there must be other provisions in place to maintain flow, otherwise the system would over pressurize and the pumps would burn out. Those provisions would hinder VFD operation and include things like bypasses that allow water to recirculate back to the chilled water plant. This is a waste of power, moving water without purpose. It also reduces the return water temperature from the buildings which makes the chillers run less efficiently. Another provision was a control valve which restricted the flow at the outlet if the pressure went above 100psi.

It wasn’t a very efficient system.

To save energy, reduce maintenance requirements, and provide better control we converted the existing motors to variable flow control with VFDs and programmed them to control to pressures measured in real time at the end of each riser. It’s important to measure the flow at the end of the riser to achieve the full savings potential. We had the pressure sensors and flow meters previously installed to help us assess the buildings demand. With the VFDs we also included passive harmonic filters and LC line rectifiers to protect the motors.

The project was implemented in February 2016 and the interesting thing about this one has been the ongoing fine tuning and adjustment. With the increased data available to us we have been able to find more opportunities for savings, improve chilled water control, and give tools to the operators to better control the buildings.

Project Cost

$ 130,000

Hydro Incentives

$ 45,000

Annual Energy Savings

$ 55,000

Project Payback

1.55 Years

Onto the technical explanation … what are motors, how do they work, and what are VFDs?

Electric AC Motors

Invented by Nicola Tesla in 1888, electric AC motors are today fundamental to our civilization. They are everywhere; they are in manufacturing, food production, and materials processing, they are in our homes, in our fridges and our furnaces, they allow us to comfortably occupy our modern buildings, and they are increasingly the future of vehicle transportation. Today these motors are responsible for approximately 45% of all electricity used in the world.

How does an AC Motor Work?

Electricity flows through opposing coils located at specific points on the stator (the part of the motor that doesn’t move), called poles (as coloured above). Because of the way the coils are wired, as the current flows through them a magnetic field is created. This field is oriented in a common direction along the axis between the coils (i.e. due to the “Right Hand Rule”). One coil thus becomes the south pole and the other becomes the north pole. The magnetic field along the axis also induces a current through the conductive metal shaft between the coils, the rotor. The induced current magnetizes the shaft, with a north–south orientation directly opposite to the field that created it. If the current was constant the motor would stay this way, locked between the stator and rotor. However, the voltage is alternating in a sine wave, moving from + to -. This causes the current to change direction, in turn causing the poles to reverse (N->S, S->N), and the rotor realigns to its new orientation. In this example 180 degrees from where it started.

When these typical 3-phase AC motors are connected to typical power panels at 600V and 60Hz, they operate at a specific speed, say 1800 rpm. The motor will run the same speed all year and will consume relatively constant power doing so. Is this necessary?

Buildings aren’t steady…..

Buildings don’t operate at steady-state equilibrium. They are constantly in flux. Occupancy numbers, weekends, holidays, seasons, daily temperature fluctuations, humidity fluctuations, sunlight hours and intensity, cloud cover, the types of activities, even how many computers and lights and other equipment are on. It all contributes to the constantly varying airflow, chilled water, hot water, and domestic water requirements. Since the demand varies, the supply should vary also. In the past this was done by restricting the flow of air on a fan or water on a pump. As discussed in the project examples that method doesn’t work very well and isn’t very efficient.

Enter Variable Frequency Drives and the Fan Affinity Laws

Instead of restricting the airflow, why not just change the speed of the motor? It turns out this is not just better for control but is a huge energy saver. This is because motor power consumption is actually proportional to the cube of motor speed. Meaning if motor speed is halved to 50% of the original speed, the power used is only 12.5% of the original power. The equations below are referred to as the fan/pump affinity laws.

In practice the savings achieved depend on the operating profile of the fan, the system curve, and control setup. For the fans I’ve worked on I’ve typically seen savings to be less than the cube, more like ~2.5 to 2.75.

The technology we most often use to run motors at different speeds is through Variable Frequency Drives (VFDs). VFDs control the power supplied to the motors to vary the speed. Sounds simple but it’s actually quite complicated. VFDs typically control power through the use of Insulated Gate Bipolar Transistors (IGBTs), to generate voltage pulses of specific duration. They convert the AC power from the building to DC power which is then turned off and on very rapidly to recreate AC power at different frequencies. By varying the amplitude of these pulses and the duration the VFDs change the power supplied to the motor, thereby varying the speed of the motor. This technique is referred to a Pulse Width Modulation (PWM).

VFDs are constantly improving and the IGBT technology will be replaced with more advanced techniques. VFDs are only going to become more prevalent as our energy needs grow, costs go up, and technology improves. Like all electronics VFDs eventually wear out but the cost savings and control benefits far outweigh the cost of the equipment, as you saw in our project examples.